One-step noble metal-aluminide coatings

ABSTRACT

A “one-step” method of forming diffused noble metal-aluminide coatings with or without minor incorporations of Si, Cr, Mn, Hf, La, and Y, is disclosed. With the inventive method, two or more powdered metals or metal alloys are applied and diffused into the metal substrate together, using a sequential multi-stage heating process. This method contrasts with the prior art technology where metals were applied and diffused into the substrate separately.

FIELD OF THE INVENTION

The present invention relates generally to aluminide coatings, and moreparticularly to a one-step process for forming diffused noblemetal-aluminide coatings.

BACKGROUND OF THE INVENTION

In the gas turbine engine industry, high temperature corrosion- andoxidation-resistant protective coatings for nickel-based andcobalt-based alloy components, such as blades and vanes, are required.These coatings are particularly useful for new generation gas turbineengines that are designed to operate at higher turbine inlettemperatures for greater engine performance and fuel efficiency.

Diffused aluminide coatings have been used to protect alloy componentsin the turbine section of gas turbine engines. In general, a diffusedaluminide coating is formed by applying an aluminum-based powder to analloy substrate and heating it to diffuse the aluminum into thesubstrate.

Diffused aluminide coatings may include chromium or manganese toincrease their hot corrosion/oxidation resistance. Furthermore, additionof noble metals, such as platinum, to provide platinum-aluminidecoatings, has markedly improved hot oxidation resistance. However,formation of these modified aluminide coatings requires additionalprocessing steps and more complex diffusion heat treatment regimes.

Specific examples of known coating processes include providing aplatinum-enriched aluminide surface by electroplating a thin film ofplatinum onto a carefully cleaned alloy substrate, overaluminizing theplatinum thin film by applying an activated aluminum-bearing coating viapack cementation, CVD, thermal spray or other known application methods,and then heating the coated substrate at a temperature and for a timesufficient to form the platinum-enriched diffused aluminum coating.Optionally, the platinum may be diffused into the substrate either priorto or after the application of the aluminum. It is also known to formthe modified diffused aluminide coating by employing a sequentialtwo-step electrophoretic deposition process with a diffusion heattreatment following each electrophoretic deposition step. (See U.S. Pat.No. 5,057,196 to Creech et al., which is incorporated by referenceherein.)

All of the known prior art processes use a multi-step applicationprocedure to sequentially apply a platinum-enriched layer and analuminum-bearing layer followed by diffusion heat treatment to providemodified noble metal-aluminide coatings. These multi-step processes areexpensive and time consuming, and make the application of such coatingsless advantageous from a commercial viewpoint. While the cost of noblemetals and chromium group metals included in the modified aluminidecoatings constitute a significant cost of the coatings, the costsassociated with the processing methods are an equally significant if notgreater cost of the coatings.

A need therefore exists for methods to streamline the noblemetal-aluminide coating processes to improve efficiency, decrease costand provide an effective corrosion- and/or oxidation-resistantprotective coating for nickel or cobalt-based alloy substrates. Thepresent invention addresses that need.

SUMMARY OF THE INVENTION

Generally describing the present invention, a “one-step” method offorming a diffused aluminide coating is provided. With the inventivemethod, two or more powdered metals or metal alloys are applied anddiffused into the metal substrate together, preferably using amulti-stage heating process. This method contrasts with the prior arttechnology where powdered metals were applied and diffused into thesubstrate separately.

A variety of powdered metals may be applied with the inventive one-stepmethod. In general, the coating compositions preferably comprise amixture of: (1) a platinum powder, and (2) an aluminum-bearingcomponent. The platinum powder preferably includes silicon either as apre-alloy powder or an alloy powder. In one embodiment, thealuminum-bearing component includes an aluminum alloy powder comprisingaluminum and chromium, although manganese is also added in somepreferred embodiments. In other preferred embodiments, an aluminumpowder is used either in addition to, or in place of, the aluminum alloypowder. In yet other preferred embodiments, hafnium, yttrium and/orlanthanum are added to one of the aforementioned powders, or are addedto the green coating composition separately.

In the coating composition described above, a portion or all of theplatinum in the platinum powder can be replaced by other noble groupmetals, for example, palladium, ruthenium, and rhodium.

Regardless of the metals used, the inventive one-step method diffusesall of the metals into the substrate together. To do that, a multi-stageheating process is preferably employed. With the multi-stage heatingprocess, the powder-covered substrate is initially heated to a firsttemperature to begin the diffusion process, and is then heated to asecond temperature to complete the diffusion. In some embodiments apre-diffusion heat treatment is also used.

One object of the present invention is to provide a simple, economicalmethod of providing diffused noble metal-aluminide coatings.

Further objects and advantages of the present invention will be apparentfrom the description provided below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1. is a graph illustrating the composition profile of a prior artdiffused platinum-aluminide coating.

FIG. 2 is a scanned image of a micrograph of one embodiment of aone-step, diffused platinum-silicon-enriched aluminide coating preparedaccording to Example 1 of this invention.

FIG. 3 is a scanned image of a micrograph of one embodiment of aplatinum-silicon-enriched aluminide diffusion coated coupon preparedaccording to Example 2 of the present invention using an electrophoreticbath.

FIG. 4 is a scanned image of a micrograph of one embodiment of anickel-alloy based coupon coated with a diffusedplatinum-silicon-manganese-enriched-aluminide coating prepared accordingto Example 3 of the present invention using a slurry coatingcomposition.

FIG. 5 is a scanned image of a micrograph of one embodiment of anickel-based alloy pin coated with a diffusedplatinum-silicon-manganese-enriched-aluminide coating prepared accordingto Example 4 of the present invention using an electrophoretic bath.

FIG. 6 is a scanned image of a micrograph of one embodiment of aplatinum-silicon-enriched-aluminide coating on a nickel-based alloyprepared according to Example 5 of the present invention.

FIG. 7 is a graph depicting the specific weight loss ofplatinum-aluminide coatings of the present invention over time whensubjected to a dynamic oxidation test.

FIG. 8 is a schematic view (partly broken away and in section) of atypical turbine blade carrying a coating of the inventive diffusedplatinum-aluminide of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

For the purposes of promoting an understanding of the principles of theinvention, reference will now be made to preferred embodiments andspecific language will be used to describe the same. It willnevertheless be understood that no limitation of the scope of theinvention is thereby intended. Any alterations and further modificationsin the described processes, coatings, or compositions and any furtherapplications of the principles of the invention as described herein arecontemplated as would normally occur to one skilled in the art to whichthe invention relates.

As briefly described above, the present invention provides a “one-step”method of forming a diffused noble metal-aluminide coating on metallicsubstrates. While this invention finds advantageous use in forming noblemetal-aluminide coatings on nickel and cobalt based substrates, it isparticularly useful for repairing and recoating metallic substrates thathave defective noble metal-aluminide coatings.

In the most preferred aspects of the present invention a single “green”coating composition (i.e. the composition that is applied to thesubstrate—before heat treatment or other curing) comprising two or morepowdered metals is applied to a metal substrate or a portion of thesubstrate having the defective coating. The coated substrate is thenheated by increasing the temperature at a controlled rate or, morepreferably, via a multi-stage heating process to form the diffused noblemetal-aluminide coating. The process provides the advantage of beingoperable at significantly reduced cost and effort when compared withconventional coating techniques.

1. Substrates

The coating compositions of the present invention can be applied to thesurface of a wide variety of substrates, with nickel- or cobalt-basedalloy substrates being most preferred. Examples of alloys that can beprotected with the noble metal-aluminide coatings according to thepresent invention include, but are not limited to: nickel-based alloyssuch as IN738, IN792, Mar-M246, Mar-M247; single crystal nickel alloyssuch as CMSX-3 or CMSX-4; and cobalt-based alloys such as Mar-M509 andX40, all of which are known to those in the art.

2. The “Green” Coating Compositions

More particularly describing the metals used in the green coatingcompositions, one embodiment uses 40-80% (by weight of the total metal)of a first powder comprising 85-100% Pt and up to about 15% Si, and20-60% of a second powder comprising 50-75% Al and 25-50% Cr. Allpercentages are listed herein weight percentages unless specifiedotherwise. A second embodiment of the present invention uses that same40-80% of a first powder comprising 85-100% Pt and up to 15% Si, andthat same 20-60% of a second powder comprising 50-75% Al and 25-50% Cr,and additionally up to 40% of a third powder comprising 95-100% Al.

In a third embodiment of the present invention the green coatingcomposition comprises 40-80% of a first powder comprising 85-100% Pt andup to about 15% Si, and 20-60% of a second powder comprising 35-45% Al,35-45% Cr, and 10-30% Mn. A fourth embodiment uses 40-80% of a firstpowder comprising 85-100% Pt and up to about 15% Si, and 20-60% of asecond powder comprising 35-45% Al, 35-45% Cr, and 10-30% Mn, andadditionally adds up to 40% of a third powder comprising 95-100% Al.

A fifth embodiment of the present invention uses a green coatingcomposition that has only the first and third powders of the earlierembodiments, and accordingly comprises 50-80% of a first powdercomprising 85-100% Pt and 0-15% Si, and 20-50% of a second powdercomprising 95-100% Al.

In alternative embodiments, a portion or all of the platinum in thefirst powder composition can be replaced by other noble metals, forexample, palladium, ruthenium, and rhodium. Alternatively, the firstpowder, the second powder, or the third powder can include up to about5% Hf, Y, La or mixtures thereof. Further, in any of the embodimentsdescribed above, the green coating composition can include up to about5% of a fourth powder comprising Hf, Y, or La or mixtures thereof,regardless of the mode of incorporation.

A summary of the embodiments described above is shown in Table 1.

TABLE 1 Coating Compositions Aluminum-Bearing Component Aluminum Alloyor Platinum Prealloy Powder Powder (wt %)² (wt %)^(1,2) 35-45 AlAluminum Powder 85-100 Pt, 50-75 Al 35-45 Cr (wt %)² Embodiment 0-15 Si25-50 Cr 10-30 Mn 95-100 Al 1 40-80 20-60 — — 2 40-80 20-60 — Up to 40 340-80 — 20-60 — 4 40-80 — 20-60 Up to 40 5 50-80 — — 20-50 ¹A portion orall of the Platinum can be replaced by other noble metals, for example,palladium, ruthenium, and rhodium. ²The metallic components can includeup to about 5% Hf, Y, La or mixtures thereof.

(a) Preferred compositions using Pt—Si powder and Al—Cr alloy powders.

As indicated above, the green coating composition may comprise about 40to about 80 wt % (based on the weight of the metal used in the coating)of a platinum-bearing powder, most preferably a platinum-silicon powder.Preferably about 55 to about 70 wt % of the platinum-bearing powder isused. In addition, the green coating compositions include about 20-60%of an aluminum-bearing component comprising aluminum and chromium metaleither as a mixture of metal powders or, preferably, an Al—Cr powderedalloy. Preferably the green coating composition includes about 30-45% ofthe aluminum-bearing component. The diffusedplatinum-silicon-enriched-aluminide coatings thus formed are generallyhigh-temperature, oxidation-resistant coatings.

For the purposes of this written description, references toplatinum-silicon powders are intended to include embodiments in whichthe amount of Pt is 100% and the Si is 0%, even though such embodimentsmight not normally be thought of as Pt—Si powders. As indicated above,the preferred embodiments include both Pt and Si.

When Pt—Si powders are used, the platinum-silicon powder can be anintimate mixture of elemental platinum and silicon, or it may be apowdered Pt—Si alloy. Preferably the platinum-silicon powder comprisesabout 85 to about 99 wt % platinum and about 1 to about 15 wt % silicon;more preferably, about 87 to about 97 wt % platinum and about 3 to about13 wt % silicon. Optionally, the platinum-silicon also can include up toabout 5% Hf, Y, La or the noble metal mixtures thereof.

The platinum-silicon alloy is preferably prepared by first mixing finelydivided platinum powder with silicon powder at about 1 micron particlesize, compacting the mixed powders into a pellet, and sintering thepellet in an argon atmosphere or other suitable protective atmosphere ina stepped-heat treatment. One such heat treatment includes sintering thepellet 1) at about 1,400° F. for 30 minutes, 2) at about 1,500° F. forabout ten minutes, 3) at about 1,525° F. for about 30 minutes, 4) atabout 1,800° F. for about 15 minutes, and then 5) at about 1,900° F. forabout 30 minutes.

The sintered pellet is then reduced to approximately an average particlesize of about 325 mesh by pulverizing in a steel cylinder and pestle andthen ball milling the pulverized particles in a vehicle (typically, 60wt % isopropanol and about 40 wt % nitromethane) for 10 to 30 hoursunder an inert atmosphere, such as argon, to produce a platinum-siliconalloy powder typically in the 1-10 micron particle size range. Suchalloy powder may also be produced by other suitable methods known in theart, such as gas atomization.

As to the aluminum-chromium alloy portion of the green coatingcompositions, the coatings preferably comprise about 20 to about 60 wt %(based on the weight of the metal used in the coating) of thealuminum-chromium prealloy or alloy powder. More preferably, the coatingcomposition includes about 30 to about 45 wt % of the aluminum-chromiumalloy. The aluminum-chromium alloy includes about 50 to about 75 wt %aluminum and about 25 to about 50 wt % chromium; more preferably, about68 to about 72 wt % aluminum and about 28 to about 32 wt % chromium.Optionally, the aluminum-chromium alloy also can include up to about 5%Hf, Y, La or mixtures thereof.

The aluminum-chromium alloy can be provided as an alloy powder preparedaccording to standard processes known in the art. Suitablealuminum-chromium alloys are commercially available. Analuminum-chromium alloy that includes about 55 wt % aluminum and about45 wt % chromium is commercially available. The powdered alloypreferably has an average particle size of about 3 to about 10 microns.

(b) Preferred compositions using Pt—Si powder, Al—Cr alloy powder, andan additional Al-bearing component.

Optionally the coating composition using Pt—Si powder and Al—Cr alloypowder can also include up to about 40 wt % of an additionalaluminum-bearing component that includes aluminum powder. Morepreferably the coating composition includes about 2 to about 20 wt % ofthe additional aluminum-bearing component.

The additional aluminum-bearing component may consist essentially ofaluminum metal powder. Alternatively, the additional aluminum-bearingcomponent may comprise at least about 95 wt % aluminum metal and up toabout 5 wt % of a metal selected from the group consisting of Hf, Y, La,and mixtures thereof. The aluminum-bearing component can be an intimatemixture of metal powders or a powdered alloy. When an aluminum-bearingcomponent is a powdered alloy, it is different in composition from theAl—Cr alloy powder discussed above.

In certain preferred embodiments the non-diffused coating compositionalso includes one or more additional metallic materials to modify thephysical and chemical properties of the coated substrate. Examples ofmetallic materials that can be included in the coating compositioninclude: Y, Hf, La, as well as and other noble metals (e.g., Pd, Rh, andRu and mixtures thereof).

(c) Preferred compositions using Pt—Si powder and Al—Cr—Mn alloypowders.

In another preferred embodiment the green coating composition comprisesabout 40 to about 80 wt % of a platinum-silicon powder, more preferablyabout 55 to about 65 wt %, and about 20 to about 60 wt % of analuminum-bearing component comprising Al, Cr and Mn metals either as amixture of metal powders or, preferably, an Al—Cr—Mn powdered alloy.More preferably, the green coating composition includes about 35 toabout 45 wt % of the aluminum-bearing component comprising Al, Cr andMn. The diffused platinum-silicon-manganese-enriched-aluminide coatingsthus formed are generally high corrosion-resistant coatings.

As with the previous embodiments, the platinum-silicon powder ispreferably a powdered alloy; although, an intimate mixture of theplatinum and silicon metals can be used in this invention. The preferredcomposition of the platinum-silicon powder is as described above.

The Al—Cr—Mn alloy is also generally as described above, although theaddition of manganese makes the preferred amounts of the various metalssomewhat different. In this embodiment, the aluminum alloy includesabout 35 to about 45 wt % aluminum, about 35 to about 45 wt % chromiumand about 10 to about 30 wt % manganese, with about 38 to about 44 wt %aluminum, about 38 to about 42 wt % chromium, and about 16 to about 22wt % manganese being more preferred. Optionally, the Al—Cr—Mn alloy alsocan include up to about 5% Hf, Y, La or mixtures thereof.

The aluminum-chromium-manganese alloy can be provided as an alloy powderprepared according to standard processes known in the art and iscommercially available. The commercially prepared powdered alloy has anaverage particle size of about 3 to about 10 microns.

(d) Preferred compositions using Pt—Si powder, Al—Cr—Mn alloy powder,and an additional Al-bearing component.

As with the case of the Pt—Si/Al—Cr alloy powder embodiments, thePt—Si/Al—Cr—Mn embodiments may also include up to about 40 wt % of anadditional aluminum-bearing component that includes aluminum powder.More preferably, about 5 to about 20 wt % of the additionalaluminum-bearing component is used.

Also as above, the aluminum-bearing component may consist essentially ofaluminum metal powder. Alternatively, the aluminum-bearing component caninclude greater than 95 wt % aluminum metal and up to about 5 wt % of ametal selected from the group consisting of Hf, Y, La, and mixturesthereof. The aluminum-bearing component can be an intimate mixture ofmetal powders or a powdered alloy. The aluminum-bearing component can beprepared by standardized processes well-known in the art, with thealuminum preferably being provided in powder form with a particle sizeof about 1 to about 10 microns.

This coating composition provides a high corrosion resistant coating fornickel- and cobalt-based alloys. However, this coating finds particularadvantages when used for nickel-based alloys.

(e) Preferred Compositions using Pt—Si powder and Al-bearing powderalone.

In yet another preferred embodiment of this invention, the green coatingcomposition comprises about 50 to about 80 wt % of a platinum-siliconpowder and about 20 to about 50 wt % of an aluminum-bearing component.More preferably the coating composition comprises about 60 to about 72wt % of the platinum-silicon powder and about 28 to about 40 wt % of thealuminum-bearing component.

The platinum silicon powder is as described above.

The aluminum-bearing component may consist essentially of aluminum metalpowder. Alternatively, the aluminum-bearing component comprises greaterthan 95 wt % aluminum metal and up to about 5 wt % of a metal selectedfrom the group consisting of Hf, Y, La, and mixtures thereof. Thealuminum-bearing component is prepared as described above.

This coating composition can be heat treated to form aplatinum-aluminide coating that exhibits high temperature oxidationresistance for both nickel- and cobalt-based alloys.

3. Application of the Coating Green Compositions

Regardless of the number or composition of the various powders used tomake the coating composition, the coating may be applied to a metalsubstrate using a variety of application methods known to the art. Theseinclude dipping, spraying, slurry deposition, electrophoretic and thelike to provide a green coating on the substrate (i.e., the compositionthat is applied to the substrate—before heat treatment or other curing).

Typically, the green coating composition is suspended in a vehicle toform a slurry, which is applied in a single application onto the surfaceof the substrate to provide a single, homogeneous, non-diffused coating.Preferred application methods include electrophoretically depositing orpainting the slurry onto the substrate surface.

The green coating composition can be electrophoretically deposited onthe nickel or cobalt-based alloy substrate after first degreasing thesubstrate and then dry-honing the cleaned substrate using 220 or 240grit aluminum oxide particles. The electrophoretic deposition step iscarried out in an electrophoretic bath that includes a vehicle, zein,cobalt nitrate hexahydrate and the desired metallic powders. A sampleelectrophoretic bath contains:

(A) vehicle comprising: 60±5% by weight isopropanol, 40±5% nitromethane;

(B) metallic powder: 20 to 35 grams total coating composition per literof vehicle;

(C) zein: 2.0 to 3.0 grams zein per liter of vehicle; and

(D) cobalt nitrate hexahydrate (CHN): 0.10 to 0.20 grams CHN per literof vehicle.

To effect electrophoretic deposition from the bath onto the nickel- orcobalt-based alloy substrates, the alloy substrate is immersed in theelectrophoretic bath and connected in a direct current electricalcircuit as a cathode. A metallic strip, for example, stainless steel,nickel or other conductive metal, is used as the anode and is immersedin the bath adjacent to the alloy substrate (cathode).

A current density of about 1 to about 2 mA/cm² is applied between thesubstrate (cathode) and the metallic strip (anode) for a time of about 1to 4 minutes, while the bath is stirred to keep the desired metallicpowders in suspension and, preferably, maintained at room temperature.During this time, a mixture of platinum-silicon powder and the aluminumcontaining alloy and/or the aluminum-bearing component are deposited asa homogenous, uniform-thickness powder deposit on the substrate surface.

The coated substrate is then removed from the electrophoretic bath andair dried to evaporate any residual solvent. The weight of the drycoating deposited on the substrate is optimally about 20 to about 40mg/cm², although coating weights from about 10 to about 50 mg/cm² aresuitable.

The coating composition also can be applied by a slurry depositionmethod to the substrate. Typically the slurry is applied by spraying,dipping or painting the substrate to provide a smooth, homogenous, anduniformly thick coating on the substrate. Good results are obtained whenthe coating is painted using a soft bristle brush.

The slurry preferably contains a mixture of isopropanol and nitromethanein a 60:40 weight ratio to suspend the powdered coating composition.However, it is understood that other vehicles that do not inhibitformation of the aluminide diffusion coating may also be used.

Most preferably, the selected vehicle maintains the metallic and alloypowders in suspension and has sufficient volatility to permit rapiddrying of the coated substrate.

Typically, the slurry contains zein (about 30 g per liter of vehicle)and about 500 to about 1000 g of the coating composition per liter ofvehicle. It is understood that the zein concentration and the coatingcomposition concentration in the vehicle are not critical to practicethis invention. Therefore, the concentration of the coating compositionand/or zein can be varied to provide a uniform coating having an optimumcoverage using a brush, a spray gun or other application equipment andmethods.

It is to be appreciated from the above that the green coatingcomposition is preferably a homogeneous mixture of the coatingmaterials. In the preferred commercial embodiments, the green coatingcomposition is prepared by mixing the various materials together beforeapplying the coating.

4. Heat Treatment to Diffuse the Applied Coatings

As indicated above, the inventive one-step method preferably uses asequential multi-stage heating process to diffuse the powdered coatingcompositions into the substrate. In the first heating stage the powderedmetal is preferably heated until it forms a transient liquid phase onthe metal substrate. To accomplish that, It is generally preferred tofirst heat the coated substrate to a temperature of about 900-1,600° F.for about 0.25 to 2 hours. More preferably, the non-diffused coatedsubstrate is subjected to a first heat diffusion treatment of about1,100 to about 1,400° F. for about 0.25 hours to about 2 hours.

In the second heating stage the coated substrate is heated sufficientlyto diffuse the coating into the substrate. Typically, the temperature israised from the first stage to the second stage in the furnace.Generally, a temperature of about 1,600-2,100° F. and a heating time ofone to eight hours is effective for that stage. More preferably, thesecond heating stage uses a temperature of about 1,850° F. to about2,080° F. and a time of about one to eight hours.

In some preferred embodiments it is advantageous to use a pretreatmentheating step as part of, or before, the first heating stage. With thismethod the first heating stage is preferably accomplished by heating thecoated substrate to a first temperature of about 950° F. to about 1,150°F. for about 0.5 to about 1.0 hours.

It is to be appreciated that the multiple heating stages may beaccomplished by “ramping” the temperature upward from the lower heattreatment temperature to the higher heat treatment temperature. Withthat technique, there may be no clear break between the first heatingstage and the second heating stage, as the two stages run smoothly intoeach other.

The diffusion heat treatment is preferably accomplished in vacuum,hydrogen, argon, or other suitable furnace atmosphere.

In one preferred embodiment the green coated substrate is subjected to apre-diffusion temperature of about 950° F. to about 1,150° F. for 0.5 toabout 1 hour. Thereafter, the coated substrate is heated to about 1,200°F. to about 1,400° F. for about 1 hour and then to about 1,900° F. toabout 1,975° F. for about 1 to about 8 hours. In another preferredembodiment the diffused platinum-aluminide coating is formed by heatingthe non-diffusion coated substrate up to a temperature of about 900° F.,and thereafter heating the coated substrate up to a temperature of about1,400° F. by judicious selection of a carefully controlled temperatureramp rate, followed by a higher temperature hold at about 1,900° F. to2,100° F. for about 1 to about 8 hours.

While not intending to be bound by any theory, it is thought that thealuminum in the aluminum-bearing material(s) melts and all othercomponents in the coating composition interdiffuse in the moltenaluminum. After sufficient time to interdiffuse the components of thecoating composition, the coated substrate is heated to a secondtemperature, higher than the first temperature, to diffuse the coatingcomposition into the substrate.

In FIG. 1 a graph of a composition profile of a typical prior artplatinum-aluminide coating is presented. This platinum-aluminide coatingis formed by electroplating a thin platinum layer on a nickel alloy(IN792), then over-aluminizing the platinum layer using an aluminum packcementation process. The microstructure is typically a dual-layerstructure having an outer layer consisting of light-etching islands ofplatinum-rich phases. The platinum content is about 35 wt % at thesurface, and the aluminum content is about 20 wt %.

For the purpose of promoting further understanding and appreciation ofthe present invention and its advantages, the following Examples areprovided. It will be understood, however, that these Examples are forillustrative purposes only, and are not intended to limit the scope ofthe claimed invention.

EXAMPLE 1 Platinum-Silicon/Aluminum-Alloy Coating

A nickel-alloy based coupon designated as Mar-M247 was cleaned by dryhoning with 220 grit aluminum oxide. A slurry coating compositioncomprised of about 1 g/ml of a mixture of 65 wt % of platinum-siliconprealloy powder (90:10; Pt:Si) and 35 wt % of an aluminum-alloy (70:30;Al:Cr) and zein (0.03 g/ml) was suspended in a vehicle comprising about60±5 wt % isopropanol and about 40±5 wt % nitromethane. After thecoating composition was brushed on the coupon, the coupon was air driedto evaporate the residual solvent. The coated coupon was heated invacuum to a first hold temperature of about 1,350° F. for one hour andthereafter heated to a second hold temperature of about 2,000° F. fortwo hours to form the diffused platinum-aluminide coating.

The coated coupon was removed from the furnace and allowed to cool toroom temperature. The coated coupon was lightly cleaned by dry honingwith 220 grit aluminum oxide.

FIG. 2 shows a scanned image of a micrograph of the one-step, diffusedplatinum-silicon-enriched-aluminide coating of Example 1. As can be seenfrom the Figure, the diffused aluminide coating is typically about 2-2.5mils thick. The nickel-based alloy substrate 10 includes a diffusedplatinum-aluminide coating 12 formed as a substantially single layer,which may include multiple zones and a diffusion zone. The diffusedcoating composition includes about 20 wt % platinum, about 4 wt %chromium, about 25 wt % aluminum and about 3 wt % silicon. Layers 14 and16 are the nickel and Bakelite layer used in the metallographicpreparation of the coupon.

EXAMPLE 2 Platinum-Silicon/Aluminum Alloy/Aluminum Metal Coating

A nickel-based Mar-M247 coupon was suspended in an electrophoretic bathcomposition. The electrophoretic bath composition contained 30 g/ml of amixture a 60 wt % platinum-silicon alloy (90:10 platinum:silicon), 30 wt% of an aluminum-chromium alloy (70:30 aluminum:chromium) and 10 wt % ofaluminum powder suspended in a vehicle that contained 60 wt %isopropanol alcohol, 40 wt % nitromethane, zein (2.0 to 3.0 g/liter) andcobalt nitrate hexahydrate (0.1 to 0.2 g/liter). The coupon was immersedapproximately equidistant between the two anodes strips. An electricalcurrent of 5.5 mA at 62 volts was applied between the substrate(cathode) and the anodes for about two minutes. During this time, agreen coating composition comprising the platinum-silicon alloy powder,the aluminum-chromium alloy and the aluminum metal was deposited as auniform coating on the substrate coupon. The coated coupon was removedfrom the bath and air dried to remove residual solvent.

The dried coated substrate was then heated in vacuum using apre-diffusion heat treatment of about 1,100° F. for about 1 hour, then afirst hold treatment of about 1,225° F. for about 1 hour, then to asecond hold treatment of about 1,925° F. for about 6 hours. Thetemperature and time of the diffusion heat treatments were selected tomelt the deposited green coating to form a transient liquid phase evenlyand uniformly covering the substrate surface. After the coated substratehad been diffusion heat treated, the coupon was cooled to roomtemperature. The coupon was then cleaned by lightly dry honing to removethe undiffused residual bisque.

A scanned image of micrograph of the platinum-enriched aluminidediffusion coated coupon is depicted in FIG. 3. The nickel-based alloysubstrate 20 is coated with a platinum-silicon-enriched diffusedaluminide coating 22. Generally, the diffused coating composition hassimilar amounts of platinum, aluminum, chromium, and silicon as inExample 1. As with FIG. 2, the nickel and Bakelite metallographic layers24 and 26, respectively, are used to prepare the sample for thephotograph.

EXAMPLE 3 Platinum-Silicon/Aluminum-Alloy Coating

A nickel-alloy based coupon designated as Mar-M247 was cleaned by dryhoning with 220 grit aluminum oxide. A slurry coating compositioncomprised of about 1 g/ml of a mixture of 60 wt % of platinum-siliconprealloy powder (90:10; Pt:Si) and 40 wt % of an aluminum-alloy(41:39:20; Al:Cr:Mn) and zein (0.03 g/ml) was suspended in a vehiclethat comprised about 60±5 wt % isopropanol and about 40±5 wt %nitromethane. After the green coating composition was brushed on thecoupon, the coupon was air dried to evaporate the residual solvent. Thecoated coupon was heated in vacuum to a first hold temperature of about1,500° F. for one hour and thereafter heated to a second holdtemperature of about 2,000° F. for two hours to form the diffusedplatinum-aluminide coating.

The coated coupon was removed from the furnace and allowed to cool toroom temperature. The coated coupon was lightly cleaned by dry honingwith 220 grit aluminum oxide.

FIG. 4 shows a scanned image of a micrograph of the diffusedplatinum-aluminide coating of Example 3. As can be seen from the Figure,the diffused aluminide coating is typically about 1.5-2.0 mils thick.The diffused coating composition contains about 1 wt % manganese inaddition to platinum, aluminum, chromium, and silicon. The nickel-basedalloy substrate 30 includes a diffused platinum-aluminide coating 32formed as a substantially single layer. Layers 34 and 36 are the nickeland Bakelite layer used in the metallographic preparation of the coupon.

EXAMPLE 4 Formation of a Modified Aluminide Coating Using a Three-StepSequential Thermal Diffusion Treatment

A nickel-based alloy (Mar-M247) pin was prepared for diffusion coatingand coated using an electrophoretic bath as described in Example 2 usingan electrophoretic bath that included the a mixture of 50 wt % of aplatinum-silicon alloy (90:10; Pt:Si), 35 wt % of analuminum-chromium-manganese alloy (41:39:20 aluminum:chromium:manganese) and 15 wt % aluminum metal.

The green-coated pin was heated under vacuum to a temperature of about1,100° F. for about 1 hour, then to a temperature of about 1,225° F. forabout 1 hour and thereafter to a temperature of about 1,925° F. forabout 6 hours.

The resulting diffused coating composition on the nickel alloy pin hadan average coating thickness of about 2.5-3.0 mils and exhibited thecharacteristic non-porous layer of an optimal platinum-aluminidediffusion coating. A scanned image of a micrograph of theplatinum-silicon-manganese-enriched aluminide coating on a pin isillustrated in FIG. 5. The nickel-based alloy substrate 40 includes adiffused platinum-aluminide coating 42 formed as a substantially singlelayer. Layers 44 and 46 are the nickel and Bakelite layers used in themetallographic preparation of the pin.

EXAMPLE 5 Platinum-Silicon/Aluminum Coating

A nickel-alloy based coupon was cleaned by dry honing with 220 gritaluminum oxide. A slurry coating composition comprised of about 1 g/mlof a mixture of 70 wt % of platinum-silicon prealloy powder (90:10;Pt:Si) and 30 wt % of aluminum and zein (0.03 g/ml) was suspended in avehicle that comprised about 60±5 wt % isopropanol and about 40±5 wt %nitromethane. After the green coating composition was brushed on thecoupon, the coupon was air dried to evaporate the residual solvent. Thecoated coupon was heated in vacuum to a first hold temperature of about1,225° F. for one hour and thereafter heated to a second holdtemperature of about 2,000° F. for two hours to form the diffusedplatinum-aluminide coating.

The coated coupon was removed from the furnace and allowed to cool toroom temperature. The coated coupon was lightly cleaned by dry honingwith 220 grit aluminum oxide.

FIG. 6 shows a scanned image of a micrograph of the one-step, diffusedplatinum-aluminide coating of Example 5. As can be seen from the Figure,the diffused platinum-silicon-enriched aluminide coating is typicallyabout 3.5 mils thick. The nickel-based alloy substrate 50 includes adiffused platinum-aluminide coating 52 formed as a substantially singlelayer, which may include multiple zones and a diffusion zone. Layers 54and 56 are the nickel and Bakelite layer used in the metallographicpreparation of the coupon.

EXAMPLE 6 Dynamic Oxidation Test

A dynamic oxidation test was performed on pins formed of Mar-M247 alloyswhich were coated with the inventive platinum-aluminide coatingsprepared in accordance to Examples 2 and 4. The dynamic oxidation testis a cyclic test in which a flame from burning JP-5 fuel impinges on arotating carousel containing the coated pins at approximately 0.3 Machvelocity. The gas temperature was measured at 2,100° F. The coated pinswere exposed to the flame for 55 minutes, then cooled by retracting themfrom the flame for 5 minutes, i.e. one cycle equals 1 hour of testing.One way of evaluating the effectiveness of the coating is to monitor theweight change per unit surface area exposed to the flame as a functionof time. Typically, the test specimens initially gain weight due tooxide formation. As the samples are cycled, the original formed oxidegets thicker and eventually spalls and additional oxide forms. Thisspallation and formation continues until more oxide spalls than isreplaced. The samples will continue to change weight and at some point,the net weight change is negative, i.e., the pins weigh less than theirinitial weight. The protection from oxidation is reduced at this time.Eventually, the oxidation rate becomes excessive and the coating is nolonger protective. As can be seen from FIG. 7, the net weight change fora bare Mar-M247 pin is negative when exposed to the dynamic oxidationtest for less than 100 hrs. However, the pins coated with the inventiveplatinum-aluminide coatings do not exhibit a net negative weight changeeven when they have been exposed to the dynamic oxidation test for morethan 400 hrs.

EXAMPLE 7 Inventive Platinum-Aluminide Coatings on Turbine Hardware

Two inventive platinum-aluminide coatings were prepared according toExample 2 and 4. These coatings were successfully applied to both anunshrouded (Mar-M247) turbine blade and a shrouded (IN-738) turbineblade. The coatings thus formed were uniform in thickness, includingalong the knife edge seals on the shrouded blade. The coatings hadmicrostructures similar to those observed on pins and coupons preparedin Example 2 and 4. This demonstrates a particularly useful quality ofthe present invention. Traditional processes for formingplatinum-aluminide coatings using electroplating techniques oftenprovide areas of non-uniform thickness coatings. The trailing edges onblades are specific areas that often exhibit formation of non-uniformthickness coatings due to high current densities on the edges.Typically, special additional fixturing is required to compensate forthe differences in current densities that occur during the platinumplating.

The inventive process avoids this problem since the electrophoreticallyapplied coating tends to be self-leveling, and the compositionsdeveloped in this invention result from a novel process where thediffused coating thickness is dependent upon the time and temperature ofthe heat treatment rather than on the application weight of the greencoat. FIG. 8 is a schematic view (partly broken away and in section) ofa typical turbine blade carrying a coating of the subject diffusedplatinum-aluminide coating. In that Figure, turbine blade 10 includes anickel- or cobalt-alloy body portion 12 provided with a diffusedaluminide coating 14 as described in this specification. For purposes ofillustration, the thickness of coating 14 is exaggerated in FIG. 8, theactual thickness being on the order of a few thousandths of an inch aspreviously described. It is usually undesirable to provide the subjectcoating over the fastening portion 16 of blade 10.

It is contemplated that processes embodied in the present invention canbe altered, rearranged, substituted, deleted, duplicated, combined, oradded to other processes as would occur to those skilled in the artwithout departing from the spirit of the present invention. In addition,the various stages, steps, procedures, techniques, phases, andoperations within these processes may be altered, rearranged,substituted, deleted, duplicated, or combined as would occur to thoseskilled in the art. All publications, patents, and patent applicationscited in this specification are herein incorporated by reference as ifeach individual publication, patent, or patent application wasspecifically and individually indicated to be incorporated by referenceand set forth in its entirety herein.

While the invention has been illustrated and described in detail in thedrawings and foregoing description, the same is considered to beillustrative and not restrictive in character, it is understood thatonly the preferred embodiments have been shown and described and thatall changes and modifications that come within the spirit of theinvention are desired to be protected.

We claim:
 1. A method of forming a diffused noble metal-aluminidecoating on a metallic substrate, said method comprising: (a) applying apowdered metal coating composition comprising about 40% to about 80% (byweight of the total applied powdered metal) of a noble metal-containingpowder, and about 20% to about 60% (by weight of the total appliedpowdered metal) of a powdered aluminum-bearing component comprisingabout 35% to about 45% aluminum, about 35% to about 45% chromium, andabout 10% to about 30% manganese, to a metallic substrate to form apowdered metal-coated substrate; (b) heating the powdered metal-coatedsubstrate to a temperature and for a time sufficient to form a transientliquid phase on the surface of the coated substrate; and subsequently(c) heating the substrate with the transient liquid phase thereon to atemperature and for a time sufficient to form a diffused noblemetal-aluminide coating on the substrate.
 2. The method of claim 1wherein said noble metal-containing powder comprises a metal selectedfrom the group consisting of platinum, palladium, ruthenium, rhodium ormixtures thereof.
 3. The method of claim 1 wherein said noblemetal-containing powder includes a platinum-silicon alloy powder thatcomprises greater than about 85% platinum and up to about 15% silicon.4. The method of claim 1 wherein said powdered aluminum-bearingcomponent comprises a powdered aluminum alloy.
 5. The method of claims 3wherein said powdered aluminum-bearing component comprises about 95% toabout 100% aluminum.
 6. The method of claim 1 comprising an additionalaluminum-bearing component comprising about 95% to about 100% aluminum.7. The method of claim 1 wherein said powdered metal coating compositioncomprises up to about 5% of a member selected from the group consistingof hafnium, yttrium, lanthanum, and mixtures thereof.
 8. The method ofclaim 5 wherein said powdered metal coating composition comprises about50% to about 80% (by weight of the total applied powdered metal) of saidplatinum-silicon alloy powder, and about 20% to about 50% (by weight ofthe total applied powdered metal) of said powdered aluminum-bearingcomponent.
 9. A method according to claim 1 wherein said heating thepowdered metal-coated substrate to a temperature and for a timesufficient to form a transient liquid phase on the surface of the coatedsubstrate comprises heating the powdered metal-coated substrate to atemperature of at least about 900° F. for a time of at least about 0.25hours.
 10. The method of claim 9 wherein said heating the powderedmetal-coated substrate to a temperature and for a time sufficient toform a transient liquid phase on the surface of the coated substratecomprises heating the powdered metal-coated substrate to a temperatureof between about 1,100° F. and about 1,400° F. for a time of betweenabout 0.25 hours and about two hours.
 11. A method according to claim 1wherein said heating the powdered metal-coated substrate to atemperature and for a time sufficient to form a transient liquid phaseon the surface of the coated substrate comprises heating the powderedmetal-coated substrate to first temperature of about 950° F. to about1,150° F. for about 0.5 to about 1.0 hours, and subsequently heating thecoated substrate to a second temperature of about 1,200° F. to about1,350° F. for about one hour.
 12. A method according to claim 1 whereinsaid heating the substrate with the transient liquid phase thereon to atemperature and for a time sufficient to form a diffused noblemetal-aluminide coating on the substrate comprises heating the powderedmetal-coated substrate to a temperature of at least about 1,600° F. fora time of at least about one hour.
 13. The method of claim 12 whereinsaid heating the substrate with the transient liquid phase thereon to atemperature and for a time sufficient to form a diffused noblemetal-aluminide coating on the substrate comprises heating the powderedmetal-coated substrate to a temperature of between about 1,600° F. andabout 2,100° F. for a time of between about one hours and about eighthours.
 14. The method of claim 1 wherein said powdered metal coatingcomposition is applied via a vehicle.
 15. The method of claim 1 whereinsaid substrate is a nickel alloy or a cobalt alloy.
 16. A method offorming a diffused noble metal-aluminide coating on a metallicsubstrate, said method comprising: applying a powdered metal coatingcomposition comprising a noble metal-silicon containing powder and apowdered aluminum-bearing component to a metallic substrate to form apowdered metal-coated substrate; heating the powdered metal-coatedsubstrate to a temperature and for a time sufficient to form a transientliquid phase on the surface of the coated substrate; and subsequentlyheating the substrate with the transient liquid phase thereon to atemperature and for a time sufficient to form a diffused noblemetal-aluminide coating on the substrate.
 17. The method of claim 16wherein the noble metal-silicon containing powder is a platinum-siliconpowder.
 18. The method of claim 16 wherein the noble metal-siliconcontaining powder is a palladium-silicon powder.
 19. The method of claim16 wherein the noble metal-silicon containing powder is aruthenium-silicon powder.
 20. The method of claim 16 wherein the noblemetal-silicon containing powder is a rhodium-silicon powder.
 21. Themethod of claim 16 wherein the noble metal-silicon containing powder isa metal alloy.
 22. The method of claim 16 wherein the noblemetal-silicon containing powder is a pre-alloy powder.
 23. The method ofclaim 16 wherein the noble metal-silicon containing powder includes upto about 5 wt %, based upon the weight of the powder a metal selectedform the group consisting of: hafnium, yttrium lanthanium or mixturesthereof.
 24. The method of claim 16 wherein said noble metal-containingpowder includes a platinum-silicon alloy powder that comprises greaterthan about 85% platinum and up to about 15% silicon.
 25. The method ofclaim 16 wherein said powdered aluminum-bearing component comprisesabout 50% to about 75% aluminum and about 25% to about 50% chromium. 26.The method of claim 16 wherein said powdered aluminum-bearing componentcomprises about 35% to about 45% aluminum, about 35% to about 45%chromium, and about 10% to about 30% manganese.
 27. The method of claim16 wherein said powdered aluminum-bearing component comprises about 95%to about 100% aluminum.
 28. The method of claim 25 comprising anadditional aluminum-bearing component comprising about 95% to about 100%aluminum.
 29. The method of claim 26 comprising an additionalaluminum-bearing component comprising about 95% to about 100% aluminum.30. A method of forming a diffused noble metal-aluminide coating on ametallic substrate, said method comprising: (a) applying a powderedmetal coating composition comprising about 50% to about 80% (by weightof the total applied powdered metal) of a platinum-silicon alloy powder,and about 20% to about 50% (by weight of the total applied powderedmetal) of a powdered aluminum-bearing component, to a metallic substrateto form a powdered metal-coated substrate; (b) heating the powderedmetal-coated substrate to a temperature and for a time sufficient toform a transient liquid phase on the surface of the coated substrate;and subsequently (c) heating the substrate with the transient liquidphase thereon to a temperature and for a time sufficient to form adiffused noble metal-aluminide coating on the substrate.
 31. A method offorming a diffused noble metal-aluminide coating on a metallicsubstrate, said method comprising: (a) applying a powdered metal coatingcomposition comprising about 40% to about 80% (by weight of the totalapplied powdered metal) of a noble metal-containing powder, and about20% to about 60% (by weight of the total applied powdered metal) of apowdered aluminum-bearing component, to a metallic substrate to form apowdered metal-coated substrate; (b) heating the powdered metal-coatedsubstrate to a temperature of between about 1,100° F. and about 1,400°F. for a time of between about 0.25 and about two hours sufficient toform a transient liquid phase on the surface of the coated substrate;and subsequently (c) heating the substrate with the transient liquidphase thereon to a temperature and for a time sufficient to form adiffused noble metal-aluminide coating on the substrate.
 32. A method offorming a diffused noble metal-aluminide coating on a metallicsubstrate, said method comprising: (a) applying a powdered metal coatingcomposition comprising about 40% to about 80% (by weight of the totalapplied powdered metal) of a noble metal-containing powder, and about20% to about 60% (by weight of the total applied powdered metal) of apowdered aluminum-bearing component, to a metallic substrate to form apowdered metal-coated substrate; (b) heating the powdered metal-coatedsubstrate to a to first temperature of about 950° F. to about 1,150° F.for about 0.5 to about 1.0 hours, and subsequently heating the coatedsubstrate to a second temperature of about 1,200° F. to about 1,350° F.for about one hour sufficient to form a transient liquid phase on thesurface of the coated substrate; and subsequently (c) heating thesubstrate with the transient liquid phase thereon to a temperature andfor a time sufficient to form a diffused noble metal-aluminide coatingon the substrate.